Multipole-multipole interactions are related to density broadening. Density broadening is very common in gasses, and those are partially caused by multipole-multipole interactions. The primary contribution to the broadening in longer ranges is the van der Waals interactions. Van der Waals interactions have been studied for over a century. The recent development in laser cooling and trapping [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]} made it possible to observe such interactions in ultracold gasses [8]}, [9]}. The primary focus of this article is the quadrupole-quadrupole interactions.
